Often, heavy polynuclear aromatic (HPNA) compounds may be a secondary byproduct from a hydrocracking process. The HPNA compounds can be a problem particularly for high conversion hydrocracking units, and be present in the reactor product. Recycling unconverted oil to increase yields of distillate product can result in an accumulation of HPNA compounds in the recycled oil. Accumulated HPNA compounds in the recycle oil may deposit on the catalyst as coke, which may degrade catalyst performance and result in shorter catalyst cycle length. Production of undesired HPNA compounds can be more pronounced for hydrocracking units processing heavier feeds. Thus, it would be desirable to remove the HPNA compounds from the unconverted oil so as to minimize the catalyst deactivation.
One way to remove HPNAs is to lower conversion by bleeding a portion of the unconverted oil to limit the accumulation of HPNA compounds. Unfortunately, this is often undesirable due to economic and logistic considerations because of yield loss and lack of market for the unconverted oil. In order to minimize the bleed rate of unconverted oil, schemes such as carbon bed absorption of the recycle oil stream to remove HPNAs and stripping columns to concentrate the HPNAs in an unconverted oil waste stream have been commercially implemented. Disposition of HPNA concentrated streams within the refinery can be problematic. If a refinery does not have a fuel oil system or an outlet for heavy bunker fuel oils, demand for which is decreasing due to increasing environmental restrictions on the use of heavy fuel oil, or an FCC unit, there may not be a good outlet for the HPNA concentrated stream.
Attempts to recirculate unconverted oil through the crude, vacuum column and coker units have in some cases led to HPNA recirculation back to the hydrocracking unit where accumulation of HPNAs led to increased fouling and catalyst deactivation. In addition, if the unconverted oil which has high HPNA content must be stored and transported to a suitable location, the storage and transfer temperatures must be kept at a level so as to prevent HPNAs from precipitating out of solution and depositing on the piping or equipment.
It would be desirable to control the level of HPNAs in the unconverted oil stream, so that it can be handled appropriately within the refinery at reduced cost.